Iron desulphurization



March 12, 1940. P, HEUER' 2,193,593

IRON DESULPHURIZAT'ION Filed Feb. 1, 1957 6 Sheets-Sheet l RCTIWTIES F SULPHUR FROM nlssocmrnon OF IRON SULPHIDE MURATE VALUES APPROXIMATE Y WULATCD N055 $0 000 I000 (ZOO I400 I 18 06 2000 o r TEMP. nasouma I w 4 1 102 65 10a 10 1 a f Z 72 a so .9 %5 96 7/01 95 74 9b 97 76 8 1 7 61 a v f M Z i a P 92 6d\. *4 ,7 I 1! I e I 1 F i ,1 i I 1 INVENTOR ATTORNEY5 March 12, 1940. HEUER 2,193,593

IRON DESULPHURIZATION Filed Feb. 1, 1937 6 Sheets-Sheet 2 S g x E E R N m famel earcefiwr- ATTORNEYS,

Mach 1940- v R. P. HEUER IRON DESULPHURIZIATION 6 Sheets-Sheet 5 Filed Feb. 1, 1937 flusifelliaiwlw' ATTORNEYi March 12, 1940.

Filed Feb. 1, 1937 6 Sheets-Sheet 5 IRON suamemum SULPHUR) (msnsm mr? sLAs (men SULPHUR) meson OXIDIZING suLPmR mow or MOI-TEN sLAs (MEDIUM SULPHUR SULPHUR woxmE sue (LOW SULPHUR) (Anson l was) IRON SLAG (MEDIUM soLPnuR) r-1 50mm) Z2 -..fi mbu 5M6 (MEDIUM SULPHUR) um:

5|.As (man suwnun) CARBON l REnucms 'ro POWDER mow Rona-nus Y (uenmm sun. 0 auLPuuR moxme PH MELI'ING Arm Row-non 0 R80 sLAs (LOW suwnun) }'*i A N (unu-n 5 Men L 1* P'WR) I I mwsmmon) a IRON sue (msnmm suwnun) m4 sum sue (men summon) 1 cannon Ala MATTE FORMING MEYAL mu m SULPHUR "i mu: oxmA'rlou Maw-rs mus menu. 1 123m HIBN \N SULPHUR I a'rnous oxlmmon w mm) 9B6 ullmow gas (MEDIUM summon) 4|.owaumun mvsm'on ATTORNEYS,

March 12, 1940. R. P. HEUER 2,193,593

IRON DESULPHURIZATION I Filed Feb. 1, 1957 e Sheets-Sheet 6 61.46 26 Tons FLUORSPAR 0-4 TONS 5M5 IRON INVENTOR meljlearwfifsw ATTORNEYS,

Patented Mar. 12, 1940 UNITED STATES PATENT OFFICE 28 Claims.

, he invention relates to the manufacture of pig iron and the production of steel, and particularly-to the desulphurlzation of pig iron.

Apparatus claims originally contained herein are included in a divisional application, Serial No. 171,800, filed October 29, 1937, for Desulphurizing apparatus.

A purpose of the invention is to operate a coke blast furnace to produce pig iron higher in sulphur than that ultimately desired, and preferably at a lower cost than normal, by operating the blast furnace at a lower temperature and/or with less basic slag and/or with low grade raw materials causing higher sulphur in the charge, and to treat the molten pig iron thus produced with a basic slag external to the blast furnace hearth, subjecting the slag and molten pig iron together to'a pressure less than normal atmospheric pressure to remove the excess sulphur from the molten pig iron. The pig iron thus produced may be used in the form of cast iron or as raw material for making steel.

A further purpose is to desulphurize molten pig iron by a basic desulphurizing slag outside the blast furnace, with the positive exclusion of air and other oxidizing substances, using a reducing agent other than carbon to produce an oxygen activity substantially lower than that obtainable under equilibrium conditions by the reaction at atmospheric pressure between carbon and oxygen to form carbon monoxide at the pig iron temerature.

A further purpose is to exclude air from the desulphurizing slag and pig iron during desulphurizing by a gasrpreferably nitrogen, whose oxygen activity is substantially lower than that of the reaction at atmospheric pressure between carbon and oxygen to form carbon monoxide under equilibrium conditions at the pig iron temperature, having a square root suitably less than 8.2x

A further purpose is to agitate molten pig iron during desulphurizing treatment with a basic slag under reducing conditions and with the exclusion of oxidizing substances while subjecting the pig iron and slag to a pressure less than normal atmospheric pressure.

A further purpose is to desulphurize molten pig iron excessively high in sulphur by treating it in a closed vessel with a basic desulphurizing slag in the presence of an excess of a noncontaminating reducing gas such as carbon monoxide under a pressure less than normal atmospheric pressure.

A further purpose is to build up a higher ratio between the sulphur in the slag and the sulphur in the pig iron than would be obtained with the same slag under atmospheric pressure.

A further purpose is to desulphurize by a slag which would not be desulphurizing at the same 5 temperature under atmospheric pressure.

A further purpose is to desulphurize an iron bath under vacuum conditions and in the presence of a slag and to subsequently protect against resulphurization of the bath during removal of 10 the slag by filling the space above the bath by a noncontaminating gas at atmospheric pressure.

A further purpose is to prevent resulphurization of a bath of molten iron which has been freed from sulphur under vacuum conditions, by filling the vacuum space with gas free from contamination by oxygen, carbon dioxide, carbon monoxide and water vapor.

A further purpose is to economize upon the use of desulphurizing slag in any of the processes disclosed herein by effecting the desulphurizing treatments in stepwise manner, preferably initially subjecting the pig iron to a first desulphurizing slag which is relatively high in sulphur content and therefore limited in its ability to take 2 up sulphur from the pig iron, and, after removal of the first slag, treating the pig iron with one or more further slags of lower sulphur content and correspondingly greater ability to remove sulphur. In this way, pig iron containing substantially more than 0.10% of sulphur, say 0.3% of sulphur or more, can have its sulphur content reduced to less than 0.05% sulphur, that is, to less than half, by treatment for an hour or more with slags which aggregate less than 8%, for example, only about 5%, of the weight of the pig iron.

A further purpose is to desulphurize molten pig iron with a slag capable of absorbing sulphur up to a percentage at least one hundred times the percentage present in the desulphurized iron, to maintain the slag and molten pig iron under a pressure less than normal atmospheric pressure, to separate the slag from the desulphurizing vessel and from the molten pig iron and to revivify the slag while itis out of contact with the molten pig iron to permit the reuse of the slag for desulphurizing more iron.

A further purpose is to withdraw molten pig iron and molten blast furnace slag from the blast furnace and to desulphurize the molten pig iron by the molten blast furnace slag under reducing conditions in a vessel maintained at a pressure below normal atmospheric pressure by a vacuum pump or the like.

A further purpose is to desulphurize molten pig iron by blast furnace slag which has been mixed with a suitable quantity of lime and desirably also with fiuorspar, conducting desulphurization in the presence of carbon with exclusion of air at atmospheric or preferably reduced pressure, or in the presence of some other reducing agent, such as a metallic reducing agent, for example metallic aluminum with exclusion of air and at any desirable pressure suitably atmospheric pressure. Where carbon is used, the carbon may in some cases be supplied by the carbon in the pig iron, although preferably the reaction should be carried out in a carbon lining, desirably in the presence of added loose carbon.

A further purpose is to desulphurize molten pig iron by a basic slag preferably containing lime in the presence of a metallic reducing agent capable of maintaining the required low oxygen activity, excluding air by a gas having a suitably low oxygen activity for example, nitrogen. Metallic aluminum is the preferred metallic reducing agent, although metallic magnesium, metallic calcium or other suitable reducing agent may be used.

A further purpose is to desulphurize molten pig iron outside the blast furnace with a lime slag and metallic aluminum, excluding air, desirably by a gas having an oxygen activity which is lower than that of the reaction between aluminum and oxygen under equilibrium conditions at the temperature of molten pig iron.

A further purpose is to conduct the desulphurization during the first stage under the influence of carbon as a reducing agent, suitably carbon dissolved in the pig iron or added carbon in the reaction vessel, and to conduct the more advanced stage of the desulphurization under the influence of a metallic reducnig agent, preferably metallic-aluminum, while excluding air throughout the desulphurization.

A further purpose is to desulphurize molten pig iron in a stepwise manner in a closed vessel under a pressure less than normal atmospheric pressure by initially treating it with a first slag relatively high in sulphur which has been used to desulphurize a previous charge of pig iron and not since revivified and by subsequently treating it with a second slag which is relatively low in sulphur, revivifying each slag out of contact with the pig iron when its sulphur content becomes high enough to prevent emcient desulphurization, In normal operation, the first slag after use is immediately revivified and the second slag after use is preserved for treatment of the next pig iron charge before revivification.

A further purpose is to desulphurize molten pig iron by a slag in such manner as to protect the slag from the influence of free oxygen, predominantly oxidizing oxides of carbon and water during the desulphurizing step whilst maintaining the molten pig iron and slag under a pressure less than normal atmospheric pressure, to subsequently treat the sulphur-bearing slag in the presence of carbon dioxide and water to remove its sulphur content and to use the treated slag to desulphurize additional pig iron.

A further purpose is to desulphurize molten pig iron by a slag in an enclosed vessel provided with means to maintain the pig iron and slag under a pressure less than normal atmospheric pressure and desirably provided with means for agitating the molten contents of the vessel.

A further purpose is to use the pig iron desulphurized in the novel manner disclosed herein I for the manufacture of steel, preferably as a continuous process in which the iron remains molten from the time it leaves the blast furnace until the time that it leaves the steel-making furnace.

Further purposes appear in the specification and in the claims.

The invention relates not only to the process involved, but also to the slag employed and to the apparatus used. The present application incorporates by reference my United States patent applications, Serial No. 20,555, filed May 9, 1935, Patent No. 2,110,066, granted March 1, 1934, for Iron and steel desulphurization, and Serial No. 96,743, filed August 19, 1936, Patent No. 2,110,067, granted March 1, 1938, for Iron desulphurization, which involve related subject matter.

The drawings are largely diagrammatic illustrations primarily intended to aid in understanding the invention, or to disclose apparatus which may be employed in performing the process. All apparatus shown is strictly schematic, and no efiort has been made to complicate the disclosure by the illustration of detail within the routine skill of those in the art.

Figure 1 is a diagram whose ordinates are the square roots of activities of sulphur from dissociation of iron sulphide. The abscissae are absolute temperatures.

Figure 2 is a diagrammatic side elevation of a desulphurizing vessel which maybe used in practicing the invention.

Figure 3 is a left end elevation of the structure of Figure 2.

' Figure 4 is a transverse section of Figure 2' upon the line 4-4 thereof, with the top opening free for admittance of material and a furnace and runner in place.

Figures 5 and 5a are fragmentary views corresponding to a portion of Figure 4 respectively without and with permissible heating electrodes and showing the opening 36 closed by an airtight cover.

Figure 6 is a chart of a desirable process of desulphurizing pig iron and revivifying the slag in accordance with the invention.

Figures 6a, 6b and 6c are charts of various processes of revivifying the slag in accordance with the invention.

Figure 7 is a diagrammatic view of apparatus which may be used for revivifying the slag.

Figure 8 is a chart of an alternate process of desulphurizing pig iron in accordance with the invention.

In the drawings like numerals refer to like parts, and in the specification like symbols refer to like subjects matter.

Throughout the specification, wherever reference is made to pig iron, it is intended to designate the product of the blast furnace which characteristically has a high carbon content, usually of 3% to 4%, or more, and always in excess of 2%, with varying contents of metalloids. The product of the blast furnace is referred to as pig iron whether it is intended to be used in the form of cast iron or whether it is to be used in making steel.

In the production of pig iron from low-sulphur burdens (such as wood charcoal and low sulphur ores) it is unnecessary to operate the blast fur- In the above slag the silica exceeds the lime plus low'sulphur ores. In most instances, however, because of the high cost of wood charcoal, it is necessary to substitute coke for wood charcoal as a blast furnace fuel. When this is done, a strong desulphurizing action must be obtained in the hearth and bush of the blast furnace in order to obtain a product having low enough sulphur to be commercially usable. For a strong desulphurizing action, it is necessary to have a higher temperature and a slag containing more lime. More fuel must be used in order to provide the higher temperature.

A typical slag from a charcoal blast furnace approximates 1 Per cent S101 47.0 A120s 18.0 CaO 27.0 MgO 3.0

magnesia. The sulphur in the pig iron approximates 0.02%. V

A typical slag from a coke blast furnace has the following approximate composition Per cent S102 34.0 A1203- 12.0 CaO 40.0 MgO 10.0 S 1.7

In the above slag the silica is less than the lime plus magnesia. The sulphur in the pig iron approximates 0.03%.

The temperatures of the slag and pig iron tapped from a charcoal blast furnace are about 1450 C. and 1410 C. respectively. In the coke blast furnace, the respective temperatures of the slag and pig iron as tapped are approximately 1525 C. and 1475" C. Thus it is unnecessary for the charcoal blast furnace to expend as much heat to produce slag and pig iron as does the coke blast fumaoe.

From the above data it will be obvious that the cost of removing sulphur from the pig iron simultaneously with smelting of the ore in a blast furnace is considerable; It is one purpose of the present invention to cheapen the cost of manufacturing pig iron, whether for use as cast iron or for making steel, by omitting the strong desulphurizing action now considered necessary in the cokeblast furnace because of the sulphur-bearing burdens. The smelting is conducted in the coke blast furnace under conditions of temperature and basicity of slag suitable to produce pig iron at lower cost but with a higher sulphur content in the iron than that ultimately desired. The sulphur content in the iron produced may be for example 0.25% sulphur. After production of this high sulphur pig iron, it is treated in novel manner as a wholly independent operation to remove the excess of sulphur. The cost of the desulphurization is small and it is possible to produce pig iron 'of desirable quality at a considerable saving. Of course it is not necessary that the high sulphur pig iron contain as much as 0.25% sulphur before treatment. The process is equally adapted to the desulphurization of pig iron containing 0.10% sulphur or 0.05% sulphur or less depending upon individual conditions.

The pig iron, containing a higher sulphur content than' that ultimately desired, is transferred from the coke blast furnace to a suitable desulphurizing vessel as described later. The pig iron is there subjected to a strongly basic slag under reducing conditions while subjecting the iron and slag to a pressure less than normal atmospheric pressure. It is generally considered that the sulphur in pig iron is present as FeS. This iron sulphide will dissociate thus:

Due to this dissociation, there is a certain activity of sulphur in the molten pig iron which is measured at constant temperature by the equilibrium constant for Equation 1, thus where (A =activity of iron (A =activity of sulphur (Am) =activity of iron sulphide K =equilibriurn constant dissociation of iron sulphide and the abscissae are temperature in degrees absolute (K). The curves are plotted for an iron-sulphur syste free from carbon.

Within the area DCEFGHI, iron sulphide and iron occur as liquid solutions. In these solutions the sulphur activity is a function of the sulphur concentration. In Figure l, diagonal dotted lines are shown corresponding to the sulphur percentagesindicated by the numerals to the right of the dotted lines. The sulphur activity resulting from a given sulphur percentage in the melt and a given temperature is shown by these dotted lines. It would require a third dimension to show the relation of sulphur activity, sulphur concentration and temperature for liquids containing iron, iron sulphide and r carbon. As an approximation for example to conditions prevailing in molten pig iron of 0.02% sulphur content at 1400 C. (1673 K.) we may extend the line for 0.02% sulphur outside the area DCEFGHI and into the area BCEFG until the temperature 1673 K. is reached and learn that at J the equals approximately 10- for 0.02% sulphur at 1400 C.

From Figure 1 it will be seen that, as the pig iron is subjected to conditions which lower the sulphur activity, the sulphur content will fall.

The desulphurizing slag contains basic oxides of the alkaline earth or alkali metals such as calcium oxide, barium oxide, strontium oxide, sodium oxide, etc. These are referred to generally as RIO 'oxides, where R is an alkaline earth or alkali metal and :r is the numeral 2 or 1, depending upon the valence of the metal. The oxides of the slag tend to react with sulphur of the pig iron thus:

For this reaction the equilibrium at constant temperature is expressed thus:

( e d-U 0 R 0)-( S 2 where r o) =activity of 11.0

( x s) =activity of R 8 (A0 =activity of oxygen K =equilibrium constant From Reaction 3 it will be evident that there will be a low activity of sulphur and sulphur can be removed from the pig iron to enter the slag most efiiciently if there is a low oxygen activity. In other words, the sulphur activity may be lowered directly by lowering the oxygen activity. The desulphurizing slag is therefore caused to act upon the pig iron in the presence of a reducing agent, as for example an excess of carbon. The carbon produces a low-oxygen activity according to the reaction:

C 1/2 02 CO (5) The equilibrium expression for this reaction is where (Ac) =activity of carbon (Aco) =activity of carbon monoxide Ka equilibrium constant The equilibrium constant K3 may be calculated from the Gibbs free energy of Reaction 5 by the equation:

AFO=RT In K: (7)

where AFo=free energy change R gas constant T=absolute temperature ln=logarithm to the base e The free energy change at a given temperature is expressed by the following equation, taken from International Critical Tables, volume VII,

page 243:

AFo:-26,6002.15 T ln T+0.00215 T At 1673 K. (1400 C.) the free energy change AFo=-6l,938. Substituting this value in Equations '7 and 6, according to the principles expressed in my United tates Patent No. 2,110,066, the oxygen activity at l400C. can be calculated to be assuming the activity of both carbon and carbon monoxide to be unity. Where carbon is the reducing agent, it is desired that the activity of carbon monoxide shall be less than unity and this desirable condition is obtained by diminishing the pressure of the reacting pig iron and slag below the normal atmospheric pressure of 760 millimeters of mercury. This pressure may be reduced, for example, to approximately '76 millimeters of mercury, thereby reducing the activity of carbon monoxide to approximately onetenth of its former value. By so doing the former value of as calculated from Equations 6 and '7 is reduced from 8.2x10- to 8.2 10"' If, for the purpose of illustration, it be assumed as in United States Patent No. 2,110,066, that it is desired to desulphurize pig iron until the sulphur activity reaches a point at which Equation 4 may be solved using this value of and the previously determined value of (A 8.2 1ofor 760 millimeters of mercury pressure, thus:

(Ag s) (8.2X 10-) Writin 2 (9) A ZZ;= 1.2 10 px2 (10 (ARZS) 1.2x 10 (ARZQKZ 11) In the present invention, using the illustration previously described for '76 millimeters of mercury pressure where Equation 11 becomes n s) X n' o) 2 The value of is a measure of the concentration or percentage of R18 in the desulphurizing slag. From Equation 12 it is evident that as the pressure on the slag and pig iron falls below the normal atmospheric pressure of 760 millimeters of mercury, the value of rises proportionally. Thus for a pressure of 76 millimeters of mercury, the lowering of pressure to one-tenth normal atmospheric pressure causes K2 high through choice of a slag of proper chemical composition, and by keeping the pressure on the slag as far below atmospheric pressure as is practical and economical. At this point it is interesting to note that the conditions attending the desulphurization of pig iron in the blast furnace hearth -are not particularly favorable because the pressure is as much as 760 millimeters of mercury or more above atmospheric pressure.

. Slags As a choice of desulphurizing slag, it is preferred to illustrate by a slag whose active desulphurizing oxide is CaO. It has previously been explained that other alkaline earth or alkali oxides can be used. The inventor has found, however, that CaO is a very desirable and economical desulphurizing agent and it is used in the illustrations, but it will be evident that the other oxides described above could be used in a similar manner after proper allowance is made for slight variations in the chemical and physical properties of the other oxides.

CaO in the pure state has a high melting point (2570 C.). It is conventional to reduce the melting point of CaO to form readily fusible slags by adding SiOz and perhaps A120: and other oxides. Thus, for example, it is possible to form a slag which is free-flowing at 1400 C. having the approximate composition: I

' v Per cent CaO 44 MgO 4 A1203 15 S102 37 In such a slag the activity of the lime is reduced by the presence of 37% silica and 15% alumina. Slags of this approximate composition are used as desulphurizing agents in the blast furnace and are capable of building up a 60:1 ratio of the percentage of sulphur in the slag to the percentage of sulphur in the pig iron. It has been found that such a slag can be used in the present invention, especially'if the slag and pig iron to be desulphurized are subjected to a diminished pressure of 150 millimeters of mercury or less. thereby causing the sulphur ratio to rise to 100:1

or 200:1 or more depending upon specific conditions.

If ordinary blast furnace slag is used, it may desirably be mixed with 5% to 10% or more by weight of fluorspar and, if desired, up to 20% or more by weight of burned lime or substances high in CaO such as burned dolomite or dolomitic lime. About 2% of lime is the minimum lime addition that would be substantial in effect.

. A typical mixture might consist of 70% blast furnace slag, 10% fluorspar, 20% lime. Such a slag when held in contact with pig iron of high sulphur content (0.16% s) at 1400 C, in a nonoxidizing atmosphere, at normal pressure, reduced the sulphur content of the iron to 0.015% S and the slag increased its sulphur content to 2.66%. A similar slag under an absolute pressure of 35 millimeters of mercury reduced the sulphur content of the iron to 0.001% and the slag contained 2.83% sulphur.

Blast furnace slag mixed with lime and/or fluorspar as just explained, may be employed in any of the forms of the invention disclosed here-1 in, whether the reducing agent be carbon at atmospheric pressure or at reduced pressure, or a metallic reducing agent such as aluminum, magnesium or calcium as later explained. Where carbon is the reducing agent, it may in some places be supplied by the carbon dissolved in the pig iron, which normally contains more than 2% of carbon, and in other cases this dissolved carbon may be supplemented by a carbon lining in the reaction vessel and/or by loose carbon added to the reaction vessel.

The SiO2 content of the desulphurizing slag can also be further decreased and slags have been synthesized containing as little as 15% $102 or less, and it has been found that such slags are very advantageous especially where the slag and pig iron under treatment cannot be subjected to low pressures.

For example a desulphurizing slag having the composition:

Per cent CaO 45 Cali: 40 S10: 15

has been found to be very satisfactory. It will be noted that this slag has the desirable features of a ratio of lime to silica of not more than 3 to 1, and a ratio of fluorspar to lime plus silica of not more than 2 to 3. Such a slag can be readily synthesized by mixing lime and fluorspar of commercial grade and adjusting the silica to the desired percentage. Magnesia present as a normal impurity in a good grade of lime is not objectionable. Alumina present as a small amount of impurity is also not objectionable. Indeed the addition of approximately 5% of alumina seems to be. advantageous as it reduces somewhat the melting point of the desulphurizing slag. The above slag flows freely at 1200 C. and this property is very desirable for eflicient desulphurization.

Molten pig iron high in sulphur (0.25%) has been subjected to the action of the lime-fluorspar-silica slag above referred to at 1400 C. for

one hour at atmospheric pressure in a refractory container composed of carbon in the form of graphite. The slag and molten pig iron were under strong reducing conditions and the deiron was reduced to 0.002% sulphur and the slag picked up 0.11% sulphur.

These good desulphurizing results can be still further improved if the pressure on the slag is reduced, for example to 380 millimeters of mercury, or below. It is to be noted, however, that low pressures such as 30 millimeters of mercury are not to be recommended with slags rich in fiuorspar unless adequate means are provided to take care of the volatile products which such slags give ofi at low pressures. The sulphur content of the iron may be reduced to less than one-..

half, or less than one-quarter, or even less than one-tenth of that. in the iron before treatment by the invention.

A study has been made of the use of soda as a substitute for lime in desulphurizing slags. The dsulphurizing action of soda, for example sodium carbonate, on pig iron is well known and this substitution can be made. It is found, however, that pure soda is readily attached by carbon at temperatures of 1400 C. or even low. producing sodium vapor which is very eifective in converting FeS into NazS and thus desulphurizing the pig iron. The sodium vapor causes difficulty in the handling of soda slags, and the reaction must be managed in such a way as to limit the formation of volatile soda derivatives in order to avoid excessive/loss as fume. It is possible to use slags containing NaaO, SiOz and A120: with other oxides to overcome volatiliz'ation loss incident to sodium carbonate. Furthermore the revivification of soda slags to remove sulphur and permit reuse presents special difllculties not present in the case of lime slags.

Detailed reference to the steps necessary when soda slags are used is therefore omitted, and the further discussion is generally confined to slags containing alkaline earth oxides as the preponderant active constituent. However, the use of slags containing soda or other alkali metal oxides as active desulphurizing ingredients, when used as substitutes for alkaline earth oxides such as lime as active ingredients to the extent that alkali sulphides are formed. instead of alkaline earth sulphides, is claimed herein.

The quantity of oxides of the type ReO in the initial slag should exceed 30% for best results.

Due to the cheapness and freedom from volatilization, it is preferable to use a lime slag, although such preference is subject to change under varying economic and metallurgical conditions.

In choosing the slag, it is desirable to have one which will be of low viscosity and workable at temperatures of 1400' C., and, for this purpose, the slag should preferably be freely fluid at a temperature as low as 1200" C. or in some cases at 1300. C.

It is very advantageous to use a slag which shows a high ratio of sulphur concentration in the slag after use to sulphur concentration in the desulphurized or partially desulphurized pig iron. If, for example, 0.25% sulphur must be removed from the pig iron and if the slag will take up as much as 12.5% sulphur, then one ton of slag will desulphurize 50 tons of pig iron.

It is to be noted that an ordinary coke blast furnaceslag can absorb approximately 1.75% sulphur and reduce pig iron to as low as 0.03% sulphur under a pressure as much as 760 millimeters of mercury above atmospheric. It has been found that such an ordinary blast furnace slag when subjected under proper conditions to approximately one-tenth normal atmospheric pressure (for example 76 millimeters of mercury pressure), becomes capable of absorbing 6% of sulphur or more and can thereby-reduce a high sulphur iron to 0.03% or less. In such a case the ratio of sulphur percentage in the slag to sulphur percentage in,.the molten pig iron equals or exceeds 200:1.

Thus by the use of diminished pressure it is possible to improve the desulphurizing power of the special synthetic slags such as the one described above containing 45% CaO, 40% CaFz, 15% SiO2, or to improve the desulphurizing power of ordinary blast furnace slags or similar slags of poorer desulphurizing ability to the point where they may be substituted-for the special synthetic slags. If slags produced in a blast furnace are to be used, the addition of approximately 5% to 10% of fluorspar and say 20% of burned lime is advantageous, as already noted.

It is obvious that the composition of the slag and the diminished pressure to be used should be chosen to meet the individual case under consideration. Much latitude is offered in theehoice of these two factors as is shown by Equations 4 and 12. In general these factors of slag composition and pressure should be chosen so that a ratio of sulphur in the slag to sulphur in the iron should exceed :1 and preferably exceed 200:1.

As explained in detail below, it is often advantageous to employ a desulphurizing slag which can be revivified or treated to remove its sulphur so that it can be used over and over again. The

. then used repeatedly to desulphurize further quantities of molten pig iron.

Desulphurz'zation The pig iron used in the process will in most cases be pig iron produced in the coke blast furnace when operated under conditions of moderate temperature and moderate basicity of the slag suitable to produce pig iron at lower cost but with a higher sulphur content than that ultimately desired. The sulphur content of the pig iron produced in the choke blast furnace under such economical conditions may be 0.25%, 0.3% or even higher. Of course the invention may also be applied to pig iron of normal sulphur content, containing say 0.04% S. The pig iron is tapped from the coke blast furnace, and, ordinarily, separated from the blast furnace slag at the time of tapping. Where the blast furnace slag is to accomplish the desulphurizing, the separation of the pig iron from the blast furnace slag at the time of tapping .is not necessary as far as the present invention is concerned. The pig iron is then preferably brought at once into contact with the desulphurizing slag, while the pig iron is still molten from the blast furnace.

The actual desulphurization may, for example, be carried out in a vessel such as that shown in Figures 2 to 5, inclusive, which illustrate a ladle car of the general type disclosed in Pugh United States Patent No. 1,534,187, granted April 21, 1925. The ladle car comprises a ladle body 20, covered with-a metallic casing 2| having a cylindrical central portion 22 and conical end portions 23 and 24 which terminate in headers 25 and 2B. The headers support trunnions 21, 21', 21 and 21 which engage bearings 28, 28', I8 and 28 The bearings 28, 28', 28 and 28 are supported from a main frame 29, which in turn-rests upon any suitable railway trucks 30 and 3|, operating upon a track 32.

To permit tilting or rocking of the ladle car, the main frame 29 is bowed at 33. The hook of a crane may be engaged with the main frame, as at 3|, to lift one side of the main frame, causing trunnions 21' to'leave bearings 28' and eventually causing trunnion 2'! to engage bearings 28 Lifting and lowering at 34' might also be used. The lifting and lowering of one side of the main frame may be used to agitate the liquid contents of the ladle car, and the lifting and lowering at al'may also be used in tapping the ladle car.

The means of rocking and tilting the ladle car need not be that shown, as any other suitable means may be employed. For example, the reaction vessel of the ladle car may be rotated by a conventional driving band and motor as shown, for example, in Hart United States Patent No. 1,916,170, granted June 27, 1933.

Inside the casing 2| is a lining 35 of suitable refractory material. It is contemplated that this will normally be graphite, or other suitable carbon refractory, although any other suitable lining material, such as magnesite or fire-clay brick, for example, might be used. If carbon be used as a lining, it may be rammed in place with a tar binder or built into the desired form from blocks which have previously been fired.

Inlet to the ladle car is provided through a charging opening 36, from a runner or launder 31 of a coke blast furnace 38. The charging opening may be closed by a door 39 which makes a gas-tight seal through gasket 40 between flanges 4| and 42.

The discharge of slag from the ladle car is facilitated by a pouring spout 43 below the charging inlet. When the pouring spout is used, the ladle car will be tilted, and the pouring spout will normally be closed when not in use by a plug 44 and a door 45 making a gas-tight seal through gasket 46 between flanges 41 and 48. Gas-tight inspection doors may be provided if desired. The ladle car is provided with a connection 49 leading to a suitable pump 49' for maintaining the desired pressure below the normal atmospheric pressure.

An atmosphere of reducing gas, primarily carbon monoxide, under a pressure below atmospheric pressure is maintained in the vessel by the pumps. The carbon monoxide comes from the reaction of any oxidizing substances with carbon present. In order to maintain this pressure the entire steel shell of the ladle and all openings are made gas tight.

It is not ordinarily necessary to heat the ladle car, as the pig iron will normally have sumcient superheat from the blast furnace to keep the contents of the desulphurizing vessel molten andto melt the desulphurizing slag when it is not charged molten. For the purpose of illustrating diagrammatically that any suitable noncontaminating heating means may be applied to the ladle car, electrodes 50 are shown in Figure 5a (one set only being visible in this figure), intended to be connected to a suitable source of electrical energy to supply are heating. No attempt in the illustration has been made to show either electrical insulation or protection against air leakage about or for the electrodes as both insulation and closure against leakage may be taken care of in various well recognized ways.- In fact the leakage incident to the electrodes need not be taken care of as the electrodes and the covers carrying them may be replaced by gastight doors after heating and before applying vacuum.

An illustrative cycle of operations is indicated in Figure '6. The specific values given on this figure vary somewhat from those mentioned in other examples, and are given as a particular instance of the process, without intention to limit the disclosure.

The ladle car containing about 2.5 tons of molten desulphurizing slag, preferably slag which has previously been used to desulphurize a previous charge of molten pig iron, is filled with about 100 tons of molten pig iron from the coke blast furnace flowing through the blast furnace runner 31, and containing as much as 0.3% sulphur or more (say 0.26% sulphur). The molten slag is held in contact with the molten ironunder the proper diminished pressure for as much as one hour or more until the sulphur content 'of the molten pig iron has fallen to between about 0.3% sulphur and 0.10% sulphur (say 0.05% sulphur) and the sulphur in the slag has increased to perhaps 5% sulphur or a much higher figure '(say 10.8% sulphur). With good operating conditions and eflicient slags, as much as 12% sulphur can be built up in the slag with only 0.4% or 0:05% sulphur in the pig iron. It is desirable to have a high concentration of sulphur in the slag when the slag is to be revivified since the amount 0 slag necessary is thereby reduced.

At this stage the slag is then removed from the ladle car by the pouring spout 43.

In the preferred process about 2.5 tons of a sulphur or even less if desired. The sulphur con--' tent of the second slag may increase from a negligible quantity at the time it is charged to 1% sulphur or 2% sulphur or more. The molten iron is then separated from the second slag, as by' tapping the second slag, then removing the molten iron, and then pouring back the second slag into the ladle car, or by retaining the molten slag in the ladle car during tapping of the iron, for example by submerging the pouring opening below the slag level before removing the plug from the pouring opening and then retaining the slag level above .the pouring opening during pouring from the ladle car.

The ladle car is then returned to a source of high sulphur pig iron to receive a further charge of say 100 tons, and the further charge is desulphurized by a first treatment with the slag which was used as the second slag on the previous charge, by removal of the high sulphur slag produced thereby, by addition of fresh slag and so on.

It will of course be understood that a stationary ladle, mixer or furnace can be employed, if it is suitably equipped to maintain sufliciently reducing conditions and suitably low pressures.

The amount of slag required per ton of pig iron desulphurized will depend on the amount of sulphur to .be removed from the pig iron and the amount of sulphur picked up by the slag. If, for example, a high sulphur pig iron containing 0.26% sulphur is desulphurized to 0.015% sulphur, then, for every 100 tons of pig iron, 0.245 tons of sulphur must be taken up by the slag. If the slag picks up 9.8% sulphur, 2.5 tons of slag will be necessary to treat 100 tons of pig iron.

In desulphurizing high sulphur pig iron, at stepwise process as described above is quite efiicient since it removes the sulphur with a small amount of slag, and necessitates regeneration of a smaller amount of slag for further desulphurization. For eflicient desulphurization the ratio of sulphur percentage in the slag to sulphur percentage in the pig iron may be as much as 250:1 or even a higher ratio. Thus a desulphurizing slag in contact with molten pig iron having 0.015% sulphur might have picked up 2.5% sulphur from the pig iron.

If 100 tons of iron were being treated to remove 0.25 tons of sulphur, 10 tons of slag might be needed if the desulphurization were done in a single step.

If the pig iron were desulphurized in a stepwise process as shown in Figure 6, reducing the sulphur first to about 0.05% sulphur and separating a sulphur-rich slag containing 10,8% sulphur, then adding a second and fresh slag to remove the balance of the sulphur in the iron down to 0.015% sulphur, then separating the iron from the second slag containing about 2.4% sulphur for use in the preliminary treatment of the next charge of sulphur-rich pig iron as above described, the removal of 0.245 ton of sulphur from 100 tons of iron can be accomplished with only 2.5 tons of slag, although 3.5 tons of slag, or more, may be used.

By stepwise treatment it is possible to reduce the sulphur content of molten pig iron from a value in excess of 0.10%, say 0.3%, to less than 0.01%, in one or two hours, by slags which aggregate less than 8% and generally not more than 5% of the weight of the pig iron.

The stepwise process of desulphurizing pig iron is not necessary however in all cases. For example, if a desulphurizing slag composed of approximately 70% molten blast furnace slag, 10% fluorspar and 20% burned lime is used, especially if used under reduced pressure, the pig iron may be properly purified with a limited amount of slag and slag expense without the stepwise treatment. One form of such treatment would involve the addition to the ladle of the required amount of molten blast furnace slag, fluorspar, lime and pig iron. The desulphurization may then be effected using preferably a reduced pressure. After the necessary purification has been effected the slag may be removed from the pig iron and discarded or revivified as desired. Such an operating cycle is shown in Figure 8.

In the example of Figure 8, tons of molten pig iron and 2.8 tons of molten slag are withdrawn from the blast furnace. into a ladle preferably of the type shown in Figures 2 to 4. To the ladle is added 0.4 ton of fluorspar and 0.8 ton of burned lime. Carbon also is desirably added, although the carbon in the pig iron and in the carbon lining are ordinarily sufiicient. The ladle is sealed and connected to the vacuum pump, the pressure being preferably less than 380 millimeters of mercury, and very desirably less than '76 millimeters of mercury.

For best results the slag used should be very liquid at the temperature prevailing. It was found that 1400 C. was an economical and desirable temperature, although higher or lower temperatures may be used. If the slag flows freely at 1200 C., it will of course be very liquid at 1400 C. 1

In the case of a lime slag, sulphur is retained in the slag as calcium sulphide. Iron sulphide present in the iron phase dissociates:

The sulphur reacts with the lime or other RIO oxide of the slag thus:

erable to desulphurize the iron while it still has its pig iron carbon content to aid in reduction than at a later stage when its carbon content has been'lowered, for example to that of steel. In

' many cases the carbon content of the pig iron itself will serve to maintain reducing conditions when air, carbon dioxide, carbon monoxide and water are excluded.

It is desirable to employ a carbon lining in the ladle car, and this serves to assist materially in maintaining reducing conditions. It is also very desirable where a carbon lining is used, and much more desirable where a carbon lining is not used, to introduce coke or charcoal into the slag, maintaining a substantial body of carbon floating on the molten pig iron. In the slag 5| floating on the molten pig iron 52 carbon is seen at 53.

When the removal of sulphur under vacuum conditions has been accomplished and before the vacuum has been released, i. e., before opportunity for resumption of normal atmospheric conditions in the desulphurizing vessel, it is desirable to insert a protecting gas under atmospheric pressure and free from contaminating quantities of oxygen, carbon dioxide, carbon monoxide and water vapor-for example nitrogen. Ifa gas, so free from contamination, be admitted under atmospheric pressure to fill up the space, resulphurization of the pig iron will be prevented during the short interval while the slag is being separated from the iron.

Reducing agents other than carbon may be used to remove the oxygen in Reaction 3 provided a sufficiently low activity of oxygen is obtained. The lower the oxygen activity, the higher the possible ratio of the percentage of sulphur in the slag to the percentage of sulphur in the iron at the end of the desulphurization, the more complete the desulphurization and the more rapid the reaction. Metallic aluminum, for example, is considered to react with oxygen in the following manner:

2Al+3/2O2 A12O3 (14) For Reaction 14, under equilibrium conditions at 1400 C. the oxygen activity is considered to be less than 10- or less than 10- In Equations 9 and 12 the oxygen activity was considered as respectively (A =8.2 l0 and Thus it is possible to obtain suitably low oxygen activity by adding metallic aluminum to the reacting slag-metal phases. If the aluminum be added to the molten iron it will dissolve in the iron. In order to economize in the amount of aluminum required, a dilute solution of aluminum in the pig iron is recommended. As this aluminum content is made more dilute, however, the reducing power of the aluminum in the solution falls. It appears that residual aluminum in amounts of 0.05% by weight in the pig iron yields a desirably strong reducing condition. Concentrations smaller than this and as low of 0.005% or less can be used depending upon the degree of desulphurization required. For each 32 parts by weight of sulphur to be removed from the iron, approximately 18 parts by weight of aluminum are required for the reaction:

In order to keep the consumption of aluminum to a minimum, the initial stage of the desulphurization can be brought about using carbon or other reducing agents, followed by the addition of aluminum to complete the final stage of desulphurization.

Metallic magnesium and calcium have the ability to combine with oxygen to produce very low oxygen activity comparable to that of aluminum. Silicon and manganese have reducing properties but the activity of oxygen which they produce, is much higher than that of aluminum, magnesium or calcium. Other reducing elements or substances may be substituted for the elements described above. Whenever metallic reducing agents are referred to herein, it is intended to include those which are metals and those which are metalloids.

The plan of using carbon as a reducing agent during the initial stages and employing a metallic reducing agent during the more advanced stages of the desulphurization may be applied with any of the metallic reducing agents. The carbon for the initial stages 'of the reaction may be supplied by the carbon dissolved in the pig iron in some cases, or by a carbon lining and/or loose carbon added to the reaction vessel, whether the reaction is carried out at atmospheric pressure or at reduced pressure. After the initial stage'of the reaction, the metallic reducing agent, for example aluminum, may be added, thus cutting down greatly on, the consumption of metallic reducing agent. Reduced pressure may be used in the more advanced stage, also, to accentuate the desulphurization action due to the carbon reducing agent present dissolved in the pig iron, or forming the carbon lining or added loose to the reaction vessel.

In cases where reducing agents such as metallic aluminum, magnesium, calcium, silicon or manganese are used, this invention teaches the use of a protecting atmosphere surrounding the reacting metal-slag phases to positively exclude oxidizing gases in order to obtain low oxygen activities. In this connection the use of nitrogen as a protecting atmosphere is recommended. Carbon dioxide may be considered too oxidizing for good results. Even carbon monoxide in high concentration may react with metallic aluminum, etc., to decrease the efiiciencyof desulphuriza- This reaction of metallic aluminum in a steel bath is well known and to it are attributed the degasifying abilities of aluminum when added to steel.

The gas used in excluding the atmosphere should have a low oxygen activity. Where a metallic reducing agent is employed, the protecting gas should be of such composition as'will provide an oxygen activity in the reacting slag and metal substantially lower than that obtainable by the reaction at atmospheric pressure between carbon and oxygen to form carbon monoxide under equilibrium conditions at the pig iron temperature.

It will thus be evident that by the present invention it is possible to accentuate the desulphurization byobtaining an oxygen activity in the reacting phase substantially lower than that obtainable by reaction at atmospheric pressure between'carbon and oxygen to form carbon monoxide under equilibrium conditions at the pig iron temperature. The square root of the oxygen activity at 00 C. will ordinarily be substantially lower than X Where carbon is the reducing agent, this low oxygen activity can be obtained by excluding the atmosphere and applying a pressure below atmospheric pressure.

Where a metallic reducing agent is to be used instead of carbon, the choice of a reducing agent, such as metallic aluminum, magnesium, calcium, silicon or manganese, and the exclusion of the atmosphere by the use of a protecting gas, insures attainment of the required low oxygen activity in the reaction vessel.

To facilitate the desulphurization, agitation of the metal and slag may be used. Such agitation may be produced by rocking or rotating the ladle .car, by raising and lowering one end of the ladle car, etc.

rated first.

may be used in any form in which the pig iron is suitably employed, as for example for gray iron or malleablized castings, etc. It is contemplated,

however, that a'large part of the desulphurized pig iron will be used in steel-making furnaces for the production of steel in much the same manner that the conventional low sulphur product of the coke blast furnace is now used.

Where steel is made, the economy in production of .the raw material will effect an over-all economy in the steel process. A further important advantage in steel making is that it will in no case be necessary to take any precautions in steel making to eliminate sulphur, as is sometimes necessary when the blast furnace pig iron runs excessively high in sulphur. This is of especial importance in the manufacture of electric steel, in which case sulphur elimination takes a substantial part of the time and contributes to the cost. Nor will it be necessary to reject certain ores or coke on account of the sulphur content when smelting pig iron for steel-making purposes.

The detail of the refining of the pig iron to make steel is not part of this invention, and the desulphurized pig iron may be used in the acid or basicopen hearth, the electric furnace, the Bessemer converter or in any other suitable manner to produce steel.

Rem'vification It has been previously explained that a wide choice of desulphurizing slags is accorded in this invention, and that special slags containing high percentages of fiuorspar or other expensive components may be used.

To utilize these expensive slags economically the invention proposesto revivify the spent slags and use them over and over again. In many cases, however, a special slag may. not be required. For example, let us consider the desulphurization of a pig iron containing about 0.10% S. One hundred tons of such pig iron could be charged into the mixer ladle with approximately 4.5 tons of molten slag taken simul-- taneously from the blast furnace. If desired 0.5 ton of fluorspar could be charged. The slag and pig iron could be maintained for example under a pressure of 76 millimeters of mercury for one hour or more. Under such conditions the sulphur in the iron would be reduced to less than 0.025% (for example to 0.02%). Such reduction would require the 5.0 tons of initial slag to absorb 0.08% of 100 tons or 0.08 ton of sulphur, the equivalent of 1.6% of sulphur in the slag.

Since the initial slag above was obtained from a blast furnace it may have contained initially about 1.5% sulphur. Thus the final slag would contain 1.5% plus 1.6% or 3.1% sulphur. This condition would require a ratio of the percentage of sulphur in the slag to the percentage of sulphur in the iron of 155:1. After desulphurization, the molten iron could be tapped from the ladle before the slag or the slag could be sepa- In separating slag and metal it is desirable to avoid any excessive oxidation-whilst the slag and metal are in contact since this will cause sulphur to leave the slag and reenter the metal. Similarly, after the desulphurization is completed it is desirable to separate slag and metal as soon as possible after the pressure has been restored to normal atmospheric pressure.

In this last illustration the desulphurizing slag need not be revivified for use over again since fresh quantities of molten blast furnace slag could be obtained more cheaply than the used slag could be revivified. The choice as to whether special slags should be used or whether the cheaper blast furnace or similar slags will suffice will depend upon individual conditions, the sul-. phur contents of the initial and final iron, the cost of revivification, etc.

There are several ways of revivifying the desulphurizing slag of the invention to lower its sulphur content so that the slag may be used again to desulphurize a further charge of pig iron, and so that the sulphur may be recovered. It is contemplated that the desulphurizing slag may be used repeatedly to treat further quantities of pig iron, with revivification when the sulphur content becomes so high that further desulphurization is impeded, and with addition of fresh slag when the losses of slag necessitate augmenting the quantity. It will benoted that, in accordance with the invention, the conditions are always reducing when the desulphurizing slag is in contact with the molten pig iron, thus minimizing the picking up of phosphorus, manganese, etc., by the slag. If the slag were in contact with the iron when oxidation was possible, pick-up of phosphorus, manganese, etc., by the slag would be excessive. Naturally, a substantial pick-up of phosphorus, for example, would render the slag unusable even though it were revivified to lower its sulphur content, and therefore it is important to avoid having contact between the molten pig iron and the desulphurizing slag under unsuitable oxidizing conditions.

The slag is revivified while it is out of contact with the molten pig iron and preferably after separation from the desulphurizing vessel, as revivification involves oxidizing reactions which would be harmful to desulphurization. I

It is preferred to effect the revivification by the principles underlying the process of Claus and Chance as used for removing the sulphur from calcium sulphide present in tank wastes produced in the LeBlanc soda process. See George Lunge, Sulphuric Acid and Alkali (D. Van Nostrand 00., N. Y., 1909) volume 2, part 2', page 943 et seq. This revivification process applies to slags containing any of the alkaline earth sulphides, but calcium sulphide, being the cheapest, is referred to below in the specific example. In accordance with this process the desulphurizing slag containing perhaps of calcium sulphide is reduced to a fine powder, preferably to a state of subdivision such that it will pass through a mesh per linear inch screen. It is of course possible to reduce the which is self-disintegrating, due to the volume changes which the alkaline earth silicates undergo when slowly cooled. The slag comprising:

Per cent CaO 45 Cal? 40 S102 15 is self-disintegrating, as it breaks up into a fine powder when cooled slowly to room temperature. The following slag is also self-disintegrating Per cent CaO CaFz 33 sum 12 The hydrogen sulphide liberated may then react Further treatment with carbon dioxide causes the reaction Ca(SH)2+COz+Hz0 CaCOa+2HzS (19) Any suitable source of carbon dioxide may be employed. As a source of carbon dioxide which is very convenient and economical at an iron or steel plant, it is preferred to use the products of combustion issuing from furnaces employing blast furnace gas as fuel. For example, the products of combustion from the hot blast stoves used in preheating the air for blast furnaces may very conveniently and cheaply be employed. It is desirable that these gases contain as much carbon dioxide as is convenient. Any sulphur dioxide in these gases should be taken intoaccount, but small quantities of sulphur dioxide are not seriously objectionable.

The products of combustion should be substantially cooled before being introduced into the slurry, to prevent difficulty through the production of steam in the revivification system.

The products of combustion are preferably blown or bubbled through the slurry in cast iron vessels such as are shown in Figure 7. Each of the vessels 54, 55, 56, 51, 58, 59 and 60 is a separate cylindrical tank about 3 feet (0.9 meter) in diameter and 15 feet (4.6 meters) tall. It is preferable to use about seven such tanks in conjunction, the exit gases from the tank receiving the gases rich in carbon dioxide passing through a series of other tanks to effect the desired reactions.

Each tank has a nitrogen cycle and a hydrogen sulphide cycle. During the operation of a given tank as the first tank in the train, the exit gases from this tank are initially nitrogen which is inert, excess carbon dioxide and hydrogen sulphide produced by Reactions 1'? and 19. These gases are passed into other tanks where both hydrogen sulphide and carbon dioxide are absorbed, until the final exit gas is substantially nothing but nitrogen, which can be exhausted.

As the'gas treatment continues, a stage is reached in one of the tanks in which the exit gases are rich in hydrogen sulphide and low in carbon dioxide. Such gases may be withdrawn and utilized for their sulphur content.

After sufiicient carbon dioxide has been absorbed in a particular tank to complete Reactions 1'7, 18 and 19, the slurry may be withdrawn from this tank and the gas rich in carbon dioxide may be applied to another tank. The slurry withdrawn from the first tank will be found to contain less than 1% of sulphur.

At any time, gas rich in carbon dioxide enters the tanks of Figure 7 through the inlet pipe SI and the exit gases leave through the outlet pipe 62. An inlet header 63 runs across all of the tanks and may be used to carry gas rich in carbon dioxide to any of them.

Each tank is equipped with a down-flow pipe 64, 65, 66, 61, 68, 69 or I0, extending from the inlet header 63 to the bottom of the tank. Each tankalso has at its top an up-flow pipe 1|, 12,

- 13, 14, 15, 16 or 11, which joins the inlet header 63. From the up-flow pipe of each tank to the down-flow pipe of the next tank is a cross-connection 18, 19, 80, 8|, 82, 83 or 84 (the cross-connection 84 is a long pipe extending across the back. of the tanks in Figure '7). The various elbows and Ts are provided with suitable cap-flanges to permit ready access to the pipes.

Numerous valves are placed to permit change in the direction of gas flow. Between the junction with each down-flow pipe and the junction with the next up-flow pipe, the inlet header 63 has a valve 85, 86, 01, 88, 89, 90 or 9I.

Between its junction with the inlet header 63 and its junction with its cross-connection, each down-flow pipe has a valve 92, 93, 94 95, 96, 9'! or 98. The inlet 6| lies between the valves 85 and 92.

Between its junction with the inlet header 63 and its junction with its cross-connection, each up-ilow pipe has a valve 99, I00, IOI, I02, I03, I04 or I05. Each cross-connection 18, 19, 00, BI, 82 or 83 has a valve I06, I01, I06, I09, H or III, while the cross-connection 84 has two ,valves H2 and I I3. Each tank is connected with the outlet 62 through a valve H4, H5, H6, H1, H8, H9 or I 20.

' If it be assumed that the tank 60 is out of serv ice, having its revivified slag removed, for example by taking off a cap-flange at the bottom, permitting air to enter at the top, and allowing the slurry to flow into any suitable drainage system,

the tanks may operate as follows. Gas high in carbon dioxide entering through the inlet 6i takes a route through valve 92 and down-flow pipe 64 into the bottom of tank 54 and bubbles up through tank 54.

A mixture of carbon dioxide, hydrogen sulphide and nitrogen may issue from tank 54. This gas is passed by up-flow pipe 1|, cross connection 18, including valve I06 in open position, and down-flow pipe 65 into the bottom of tank 55.

Most of the carbon dioxide and hydrogen sulphide are absorbed in tank 55, but the gas is passed through up-flow pipe 12, cross-connection 19, including valve I01 in open position, and down-flow pipe 66 into the bottom of tank 56. In tank 56 the remainder of the carbon dioxide and hydrogen sulphide is absorbed, and the effiuent gas is substantially nitrogen, which passes through valve II6 into outlet 62. During the nitrogen cycle this eiiiuent is allowed to escape.

'As the blowing of tank 54 continues, more hydrogen sulphide passes over into tanks 55 and 56, and eventually the quantity of hydrogensulphide in the eflluent gas from tank 56 becomes appreciable. At this point, the efiluent gas is led through tank 51'by closing valve H6 and directing the gas through up-flow pipe 19, cross-connection 80 (valve I08 in open position) and down-flow pipe 61 into the bottom of tank 5?, and then by outlet valve II'I into outlet 62. With increase in the hydrogen sulphide content of the effluent gas from tank 51, it may be desirable to add, tank 59 to the train by closing outlet valve H1 and openingcross-connection valve Hi9 and outlet valve H8.

As the blowing of tank 54 proceeds further, the eflluent gas from tank 56 (which is being carried to tanks 51 and 58) reaches such a high concentration of hydrogen sulphide that it can be utilized to recover the sulphur. This is the end of the nitrogen cycle and the beginning of the hydrogen sulphide cycle. Tanks 51 and 58 are cut out of the train by opening outlet valve H6 and closing cross-connection valves I08 and I09 and outlet valve II8. The outlet 62 is connected to suitable storage or recovery mechanism. As the blowing of tank 54 continues still further, the hydrogen sulphide content of the eflluent gas from tank 56 decreases and carbon dioxide begins to come over into the eiiluent gas from tank 56. This is the end of the hydrogen sulphide cycle and the beginning of another nitrogen cycle. Other tanks are then put back in train, for example by opening valves I08, I09 and H0, and closing valve II6. Finally the contents of tank 54 are completely revivified and gas rich in carbon dioxide is led to tank 55 by closing valves 92 and I06 and opening valves 85 and 93. Tank 54 can now be emptied and refilled with slurry to be revivified. At the proper times in the cycle for tank 55, tank 59 is added to the train. In general, other valves not mentioned during the above discussion are kept closed until it is necessary to open them when other tanks are in service.

The above discussion is merely illustrative of one manner of using the tanks, and is not intendedto limit the disclosure, as other apparatus may be employed or this apparatus may .be used in other ways.

- The revivified slag can be removed from the tank in which the process is completed, filter pressedor run through a Dorr thickener or similar apparatus, dried and used for further desulphurization of additional charges of pig iron. The drying may be done in a rotary drier if desired and the charge issuing from the drier may attain temperatures approximating 1000 C. At these temperatures, calcium carbonate is changed to calcium oxide. This hot discharge material can be placed in suitable containers to conserve its heat content and used as a desulphurizing slag and a first slag on the next charge, before it is revivified. It would of course be possible to use the same slag three or a. greater number of times before revivification, but this would complicate the process. In any case, when the sulphur content of the slag reaches a predetermined value, the slag is separated from the molten pig iron and revivified.

The gas rich in hydrogen sulphide obtained from revivification may be used for its sulphur content.

the sulphur dioxide may be made into sulphuric acid or other suitable compounds. Or, in the presence of a suitable catalyst, the hydrogen sulphide may be burned directly to sulphur trioxide. The hydrogen sulphide may also be converted into elemental sulphur by combustion with the For example, it may be burned to sulphur dioxide by mixing it 'with sufiicient air, and

required amount of air in accordance with the following reaction:

Whether the sulphur be changed to sulphuric acid, recovered as elemental sulphur or in some other form, the value of the resulting product may be credited against the cost of revivifying the slag, and may in some cases more than pay for the cost of revivification.

Figure 6 shows, by way of example, a series of steps which may be employed in the process. The ladle shown at the top of the chart may suitably contain tons of molten pig iron of say 0.26% sulphur content. Into the ladle is charged about 2.5 tons of a first lime-fiuorspar desulphurizing slag containing about 2.40% sulphur. This high sulphur content is due to previous desulphurizing use of, the slag subsequent to revivification. After suitable contact between the molten pig iron and the first desulphurizing slag in the ladle, the slag is separated from the pig iron. The first slag now contains, for example, 10.8% sulphur.

The slag is now ground to say 50 mesh per linear inch or allowed to self-disintegrate by slow cooling and pre-selection of the proper composition. The ground slag is then mixed with water to form a slurry and treated with a gas high in carbon dioxide, by which the slag is revivified. and hydrogen sulphide gas driven off. The hydrogen sulphide gas may be burned in a suitable furnace to liberate sulphur, or, in the alternative, to form sulphur dioxide. The revivified slurry is next dewatered'and calcined to produce a revivified slag containing, say 1% of sulphur.

The revivified slag is then supplied to a subsequent charge in the ladle and there used as a second slag for treating pig iron which was initially desulphurized with a previous slag. The pig iron before treatment with the second slag may have a sulphur content of say 0.05% sulphur, but after treatment with the second .slag its sulphur content will drop to perhaps 0.015% sulphur. The sulphur content of the slag, in the meantime, will increase from about 1% to about 2.40%, This second slag is then available for use as a first slag to treat a new charge of pig iron.

Through both of the desulphurizing treatments, the ladle is maintained under reducing conditions by the presence of carbon and the positive exclusion of the atmosphere and combustion gases high in carbon dioxide and water vapor. The mixture of oxides of carbon present in the ladle is predominantly reducing due to the excess of carbon.

It is contemplated that there will be certain losses of slag during the desulphurizing process, and that certain small amounts of impurities may be picked up, both of which features will necessitate additions of fresh slag-making materials from time to time. By the use of proper refractories in the ladle car, by the maintenance of reducing conditions when the slag is in contact with the molten pig iron, and by suitable ing the powdered slag at higher temperature with steam and carbon dioxide gas, instead of carrying out the process by treating a slurry at moderate temperature.

There are a number of other processes by which revivification may be accomplished. For example, as indicated in Figure 6a. sulphur may be removed from the slag directly as sulphur dioxide gas, by maintaining the high-sulphur slag molten and at high temperature after it is removedfrom the ladle car and blowing air or oxygen through the slag. In this way sulphur dioxide will form readily and the sulphur content of the slag may be effectively reduced.

- Another process of revivifying the slag, as shownin Figure 6b, is to allow it to cool, and reduce it to a fine powder, as by self-disintegration or by crushing and grinding. A portion of the slag is then roasted in a conventional furnace used for roasting sulphides, desirably to a temperature of about 1000 C. Calcium sulphide is thereby changed to calcium sulphate and some of the sulphur is driven off as sulphur dioxide.

The slag containing calcium sulphate is then mixed with a theoretical quantity of unroasted slag and melted to cause calcium sulphide and calcium sulphate to react:

This reaction is carried out with the slag in molten condition at about 1200 C to 1400 C. In this way the greater part of the sulphur from the sulphur-rich slag is driven off as sulphur dioxide, and may be used for manufacturing sulphuric acid or in other suitable manner.

A serious difliculty with both of the last-mentioned processes for revivifying the desulphurizing slag is that they require that the slag be maintained molten under oxidizing conditions. Slags high in lime and calcium fluoride are rather corrosive to refractories other than carbon and, while carbon may be used in the ladle car under reducing conditions, it may not be used in operations involving the volatilization of sulphur dioxide where oxidizing conditions are necessary. It is therefore obligatory to have recourse to magnesite and similar refractories, and magnesite brick may be attacked by the slag.

Another process for revivifying the slag, as indicated in Figure 6c, involves the conversion of the sulphur in the slag to a metal sulphide capable of forming a molten phase immiscible with the molten slag, known as a matte. Copper is a suitable matte-forming metal; nickel might also be used. A sulphide of the type RXS reacts with a matte-forming metal thus Molten slag is brought into contact with molten copper preferably at about 1200 C. and sufficient oxidation is brought about, as for example by blowing air mildly into the copper. Cuprous sulphide forms and enters the layer of copper below the slag. The revivified slag may subsequently be separated from the matte and used for desulphurizing a further charge of pig iron.

The sulphur picked up by the copper may be eliminated by intensive oxidation of the matte through vigorous blowing with air, as for example in the conventional copper matte converter:

After removal of the sulphur from the copper, the copper bath may be used to desulphuriz more slag.

The decision as to whether to revivify the slag by a wet process at low temperatures to eliminate hydrogen sulphide or by a dry process at high temperatures will depend upon the factors prevailing in individual cases. It is considered, however, that the wet process will be more economical in many instances.

Numerous examples have been given herein to aid in practicing the invention. It is not intended, however, to limit the disclosure by reason of the inclusion of these examples except where limitations are included in the claims or indicated by the specification to be essential.

Where reference is made herein to atmospheric pressure, it is intended to designate the prevailing atmospheric pressure.

It will be evident that, by exposing the pig iron to'the desulphurizing slag under a pressure below atmospheric pressure for a sufiicient duration, a higher ratio between the sulphur in the slag and the sulphur in the pig iron can be built up than would be obtained with the same slag under atmospheric pressure. Thus underreduced pressure the same desulphurization may be obtained with a smaller quantity of slag than under atmospheric pressure, or under reduced pressure more complete desulphurization may be obtained than under atmospheric pressure with the same quantity of slag, or desulphurization maybe obtained under reduced pressure with a slag which would not desulphurlze or might even sulphurize under atmospheric pressure.

It will also be evident that the rate of desulphurization by a given slag is increased where the pressure is reduced below atmospheric pres- ,sure, so that the time required-to produce a given phere by a suitable gas, or the reaction vessel may be maintained under reduced pressure to accentuate the reduction from the carbon present and supplement the action of the metallic reducing agent.

Where slag compositions are given, it will be understood that these are intended as compositions of the slag effective for desulphurization during a particular stageof the treatment, and

not necessarily as the compositions present prior to or subsequent to this efiective portion of the desulphurizing operation.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the process or structure shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The process of desulphurizing pig iron, which comprises treating molten pig iron outside the blast furnace and without application of additional heat under substantial exclusion of air, with a basic desulphurizing slag in the presence of a metallic reducing agent which produces an oxygen activity in the iron and slag substantially lower than that obtainable under equilibrium conditions by the reaction between carbon and oxygen to form carbon monoxide at the pig iron temperature and atmospheric pressure.

2. The process of desulphurizing pig iron, which comprises treating molten pig iron outside the blast furnace, without application of additional heat and with substantial exclusion of air, with a basic desulphurizing slag containing lime in amounts not exceeding in the presence of a metallic reducing agent which produces an oxygen activity substantially lower than that ohtainable under equilibrium conditions by the reaction between'carbon and oxygen to form carbon monoxide at the pig iron temperature and atmospheric pressure.

3. The process of desulphurizing pig iron, which comprises treating the pig iron in molten condition outside the blast furnace with a basic mixture of blast furnace slag and added limejproduc-ing a total lime content of less than 55%, in the presence of a reducing agent and with the substantially complete exclusion of air.

4. The process of desulphurizing pig iron, which comprises treating the pig iron in molten condition outside the blast furnace with a basic mixture of blast furnace slag, added lime and fiuorspar, producing a total lime content of less than 55%, in the presence of a reducing agent and with the substantially complete exclusion of air.

5. The process of desulphurizing pig iron, which comprises withdrawing pig iron from a blast furnace, withdrawing a basic slag from a. blast furnace, mixing with the blast furnace slag between about 2% and 20% by weight of burned lime and between about 5% and 10% by weight of fiuorspar and treating the pig iron while still molten with the mixture of blast furnace slag. lime and fiuorspar, in the presence of a reducing agent and with the substantially complete exclusion of air.

6. The .process of desulphurizing pig iron,

which comprises treating the pig iron in molten condition in a vessel outside the blast furnace and without application of additional heat, with a basic mixture of blast furnace slag and added lime, producing a total lime content of less than 55%, in the presence of carbon and maintaining a pressure in the vessel below atmospheric pressure.

'7. The process of desulphurizing pig iron,

which comprises treating the pig iron in molten condition in a vessel outside the blast furnace and without application of additional heat, with the pig iron, while still molten, with the mixture of blast furnace slag, lime and fiuorspar in the presence of carbon and maintaining a pressure below atmospheric pressure, with exclusion of air.

9. The process of desulphurizing pig iron, which comprises treating molten pig iron containing in excess of 2% of carbon, outside of the blast furnace, with a basic slag mixture predominantly containing blast furnace slag, with added lime 1 and fluorspar as minor constituents present in effective quantities, the total lime content of the slag being less than and maintaining a pressure below 380 millimeters of mercury with exclusion of air and other oxidizing substances.

10. The process of desulphurizing pig iron, which comprises treating the pig iron in molten condition outside the blast furnace with a basic slag comprising blast furnace slag, added lime in quantity suflicient to increase the ratio of lime to silica in the slag to not more than 3 to l, and added fluorspar in quantity suflicient to increase the ratio of calcium fluoride to the sum of the lime and silica in the slag to not more than 2 to 3 in a carbon lining with the exclusion of air and other oxidizing substances.

11. The process of desulphurizing pig iron, which comprises treating the pig iron in molten condition outside the blast furnace with basic slag comprising blast furnace slag, added lime in quantity sufficient to increase the ratio of lime to silica in the slag to not more than 3 to 1, and added fiuorspar in quantity sufiicient to increase the ratio of calcium fluoride to the sum of the lime and silica in the slag to not more than 2 to 3 in a carbon lining with the exclusion of air and other oxidizing substances. 1

12. The process of desulphurizing pig iron, which comprises treating the pig iron in molten condition outside the blast furnace with a basic mixture of blast furnace slag and added lime, the total lime content of the slag being less than 55%, in a carbon lining with the exclusion of air and other oxidizing substances and maintaina pressure substantially below atmospheric pressure.

13. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a lime desulphurizing slag in the presence of a metallic reducing agent which produces in the slag and iron an oxygen activity substantially lower than that obtainable under equilibrium conditions by the reaction between carbon and oxygen to form carbon monoxide at the pig iron temperature and atmospheric pressure while agitating the contents of the vessel and positively excluding air is substantially less than 8.2 x 10- and positively excluding air by a gas having an oxygen activity whose square root is also substantially less than 8.2 x 10- 15. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace and without application of additional heat, with a basic desulphurizing slag comprising lime and another oxide of the type RIO, where R is an element of the alkali or alkaline earth group and a: is the numeral 2 or 1 depending upon the valence of R, in the presence of a metallic reducing agent which produces an oxygen activity in the slag and iron substantially lower than that obtainable under equilibrium conditions by the reaction between carbon and oxygen to form carbon monoxide at atmospheric pressure and the pig iron temperature; and positively excluding air from thefvessel.

16. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a basic desulphurizing slag and aluminum to produce an oxygen activity in the slag and iron substantially lower than that obtainable under equilibrium conditions by the reaction at atmospheric pressure between carbon and oxygen to form carbon monoxide at the pig iron temperature, and positively excluding air from the vessel by a suitable protecting gas.

17. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a basic desulphurizing slag and magnesium to produce an oxygen activity in the slag and iron substantially lower than that obtainable under equilibrium conditions by the reaction at atmospheric pressure between carbon and oxygen to form carbon monoxide at the pig iron temperature, and positively excluding air from the vessel by a suitable protecting gas.

18. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a basic desulphurizing slag and calcium to produce an oxygen activity in the slag and iron substantially lower than that obtainable under equilibrium conditions by the reaction at atmospheric pres sure between carbon and oxygen to form carbon monoxide at the pig iron temperature, and positively excluding air from the vessel by a suitable protecting gas.

19. The process of desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a lime desulphurizing slag in the presence of more than 0.005% metallic aluminum by weight in the pig iron, and protecting the contents of the vessel from air, building up in the vessel an oxygen activity less than that possible with carbon under atmospheric pressure.-

20. The process 01' desulphurizing pig iron, which comprises treating molten pig iron in a vessel outside the blast furnace with a lime desulphurizing slag in the presence of more than 0.005% metallic aluminum by weight in the pig iron, and excluding air from the vessel by a suitable protecting gas, building up in the vessel an oxygen activity less than that possible with carbon under atmospheric pressure.

21. The process of desulphurizing pig iron, which comprises treating molten pig iron outside the blast furnace with .a basic desulphurizing slag in the absence of air and other oxidizing substances, in the presence of carbon during the initial stage of the desulphurization and in the presence of metallic aluminum during the more advanced stage of the desulphurization.

22. The process of desulphurizing pigiron, which comprises treating molten pig iron outside the blast furnace with a lime desulphurizing slag in the absence of air and other oxidizing substances, in the presence of carbon during the initial stage of the desulphurization and in the presence .of metallic magnesium during the more .advanced stage of the desulphurization, the magnesium being added at the beginning of the more advanced stage.

23. The process of desulphurizing pig iron, which comprises treating molten pig iron outside the blast furnace with a basic desulphurizing slag with the positive exclusion of air and other oxi- 

